Quaternary stratigraphy of Bayan borehole in eastern Songnen Plain and its paleoclimate significance

The Songnen Plain is a key region for Quaternary stratigraphic research in Northeast China. In this area, the Quaternary strata of the Bayan borehole reached a thickness of 58.00 m, offering excellent potential for investigation and research. A comprehensive analysis of the borehole was conducted using lithological characteristics, optically stimulated luminescence (OSL), electron spin resonance (ESR), magnetic susceptibility, grain-size distribution, and magnetostratigraphy. Based on these results, a polarity stratigraphic framework was established, subdividing the sequence into four units: the Holocene Series (0-1.50 m), Guxiangtun Formation (1.50-18.80 m), Harbin Formation (18.80-35.50 m), and Huangshan Formation (35.50-58.00 m). Chronostratigraphic and paleoenvironmental interpretations were derived to establish a Quaternary stratigraphic framework for the eastern Songnen Plain. Grain-size end-member analysis was applied to the Bayan borehole using the Analysize method to extract three effective grain-size components. EM₁ (6.72 μm) corresponded to the atmospheric background dust; EM₂ (21.20 μm) reflected the input from distal dust sources associated with southwesterly wind signals; and EM₃ (45.60 μm) indicated the proximal deposition influenced by winter monsoon intensity. Comparative analysis of magnetic susceptibility (MS), mean grain size (M_z), grain-size end-members (EM), and marine isotope stages revealed strong correlations, identifying multiple climate response events. In the eastern Songnen Plain, the Middle Pleistocene was marked by a transition from dry-cold to cool-moist conditions, with a boundary at approximately 250 ka. During the Late Pleistocene, the region experienced alternating phases of dry-cold and cool-semi-humid conditions. The Holocene was characterized by a warm and semi-humid climate. These findings provide a robust data foundation for refining the Quaternary stratigraphy of the Songnen Plain and offer critical insights into its formation history, stratigraphic evolution, and paleoclimatic development.